The upper portion of 39 is used only for emergency vehicles and as stated is down to one lane in many places. The USFS Big Pines Station patrol area, Area 38, extends on Hwy 2 to Islip Saddle. When responding to "motorcycle down" calls on the far western portion of our area on Hwy 2, we often find that USFS personnel from the San Gabriel River District, Crystal Lake area, have beaten us there, their having used Hwy 39 to reach Hwy 2.
This upper portion of 39 will most likely never be opened to the public, as it is far too unstable as already mentioned and any "fix" would only prove temporary as the road would AGAIN be washed away in the next big storm. It has been repaired and reopened numerous times over the last 70 years and always with the same result. It was last closed back in the 70s. Hwy 39, like Hwy 2 is maintained by CalTrans, NOT the USFS. Despite pressure from politicians in Azusa, the State, CalTrans, has elected not to throw massive amounts of money into the proverbial "bottomless pit". It is kept passible for emergency vehicles only and could be used as an emergency evacuation route during a very serious fire event.
As mentioned the closure only affects vehicles and is open to hiking and biking, though again, you might want to think twice about those boulders that routinely come free from the mountain...